fix: restore saved per-satellite calculator offset on open

The per-satellite doppler offset was loaded only inside SelectTransmitter, so
it never came back when the radar page was reopened, and it was saved under
the selected transceiver catnum - null after toggling the selection off, which
dropped the write silently. The CalculatorPage chips that trigger
SelectTransmitter are only rendered when a satellite has more than one linear
transponder after deduplication, so single-transponder satellites (JO-97,
RS-44, FO-29, AO-73, QO-100, TO-108, FO-82, QB50P1, UKUBE-1) had no way to
load it at all.

The offset now follows the transponder driving the calculator: it loads when
that transponder changes and saves under its catnum regardless of selection.

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/RadarState.kt

@@ -101,4 +101,11 @@ sealed interface RadarAction {
 
     // Calculator actions
     data class ChangeCalculatorOffset(val offsetKHz: String) : RadarAction
+
+    /**
+     * The transponder currently driving the calculator (null when the satellite has
+     * no linear transponder). The per-satellite offset follows it instead of the
+     * transceiver selection, so it loads and saves even when nothing is selected.
+     */
+    data class CalculatorTransponderChanged(val catnum: Int?) : RadarAction
 }

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/RadarViewModel.kt

@@ -70,6 +70,7 @@ class RadarViewModel(
     private var compassOffset = settingsRepo.otherSettings.value.radarCompassOffset
     private var compassOffsetElev = settingsRepo.otherSettings.value.radarCompassOffsetElev
     private var transponders: List<SatRadio> = emptyList()
+    private var calculatorCatnum: Int? = null
     private var sstvDecoder: SstvDecoder? = null
     private var sstvRecordingJob: Job? = null
     private var sensorCollectionJob: Job? = null
@@ -260,16 +261,8 @@ class RadarViewModel(
                 // Compute toggle state before the update so we don't read post-update value
                 val isTogglingOff = _uiState.value.transceivers.selectedUuid == action.uuid
                 val newUuid = if (isTogglingOff) null else action.uuid
-                // Load the saved offset for the newly selected satellite
-                val offsetKHz = if (!isTogglingOff) {
-                    transponders.find { it.uuid == action.uuid }
-                        ?.catnum?.let { settingsRepo.getSatelliteOffset(it) } ?: ""
-                } else ""
                 _uiState.update {
-                    it.copy(
-                        transceivers = it.transceivers.copy(selectedUuid = newUuid),
-                        calculatorOffsetKHz = offsetKHz
-                    )
+                    it.copy(transceivers = it.transceivers.copy(selectedUuid = newUuid))
                 }
                 // Only update the tracking service when selecting a different transponder to
                 // avoid resetting a user-adjusted TX base on re-expand
@@ -320,7 +313,9 @@ class RadarViewModel(
                 _uiState.update { it.copy(sstv = it.sstv.copy(currentFrame = null)) }
             }
             is RadarAction.ChangeCalculatorOffset -> {
-                val catnum = _uiState.value.transceivers.selectedUuid?.let { uuid ->
+                // Keyed by the transponder driving the calculator, so the offset saves
+                // even when no transceiver is selected (or selection was toggled off)
+                val catnum = calculatorCatnum ?: _uiState.value.transceivers.selectedUuid?.let { uuid ->
                     transponders.find { it.uuid == uuid }?.catnum
                 }
                 if (catnum != null) {
@@ -328,6 +323,11 @@ class RadarViewModel(
                 }
                 _uiState.update { it.copy(calculatorOffsetKHz = action.offsetKHz) }
             }
+            is RadarAction.CalculatorTransponderChanged -> {
+                calculatorCatnum = action.catnum
+                val offsetKHz = action.catnum?.let { settingsRepo.getSatelliteOffset(it) } ?: ""
+                _uiState.update { it.copy(calculatorOffsetKHz = offsetKHz) }
+            }
         }
     }
 

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/TransceiversPage.kt

@@ -140,6 +140,13 @@ fun CalculatorPage(
             ?: calculatorTransceivers.firstOrNull()
     }
 
+    // Keep the per-satellite offset in sync with the transponder driving the
+    // calculator: load the saved value on open/satellite change, and remember
+    // its catnum so editing saves even when no transceiver is selected
+    LaunchedEffect(selectedTransceiver?.uuid) {
+        onAction(RadarAction.CalculatorTransponderChanged(selectedTransceiver?.catnum))
+    }
+
     if (selectedTransceiver == null) {
         EmptyTransceiversContent(modifier)
         return